The present invention relates generally to an optical system for recording or replicating holograms. More specifically, the present invention relates to a hologram recording or replicating optical system capable of preventing misalignment of the center of a laser generated beam with the passage of time, which may otherwise disturb a color balance in a color hologram surface, and a multicolor hologram recording or replicating optical system which can prevent a variation of the color balance in a multicolor hologram surface, which is caused by a laser beam diameter difference for each color.
So far, R (red), G (green) and B (blue) full-color hologram has been recorded by means of an optical system as typically shown in FIG. 6 that illustrates an example of recording a full-color Lippmann hologram (reflection hologram). In this example, a photosensitive material 20 such as a photopolymer is used with light sources, an R laser 1 (e.g., 647-nm Kr laser), a G laser 2 (e.g., 576-nm dye laser) and a B laser 3 (e.g., 458-nm Ar laser). To synthesize laser beams from these lasers into one optical path, a total reflecting mirror 4 and dichroic mirrors 5 and 6 are used. In the illustrated arrangement, the dichroic mirror 5 is a red narrow-band mirror having a non-reflective coating on its back surface, and the dichroic mirror 6 is a mirror that has a non-reflective coating on its back surface and selectively reflects only light having a wavelength of 500 nm or greater. The lasers 1 to 3 are not necessarily located according to the illustrated layout, and so may be located at different positions. In this case, however, it is required to alter the positions and reflection bands of the total reflecting mirror 4 and dichroic mirrors 5 and 6.
Light coming from the RGB three-colors lasers 1 to 3 and synthesized through the total reflecting mirror 4 and dichroic mirrors 5 and 6 is split by a half-mirror 7 into two ray bundles, one of which is focused through a mirror 8 and a lens 9 to a pinhole 10. Divergent light leaving the pinhole 10 is obliquely incident on one side of the photosensitive material 20. On the other hand, the other ray bundle is focused through mirrors 11 and 12 and a lens 13 to a pinhole 14. Divergent light leaving the pinhole 14 is incident on the other side of the photosensitive material 20. Then, both divergent ray bundles interfere with each other in the photosensitive material 20, so that the hologram of an object illuminated with the divergent light leaving the pinhole 14, for instance, can be recorded therein.
For instance, to use a RGB primary-colors full-color hologram plate thereby replicating a similar hologram therefrom, such an optical system as shown in FIG. 7 is used. In the FIG. 7 embodiment, a full-color Lippmann hologram (reflection hologram) is replicated as an example. As replicating illumination light sources, an R laser 1, a G laser 2 and a B laser 3 are used as in FIG. 6. Laser light rays from these are synthesized through a total reflecting mirror 4 and dichroic mirrors 5 and 6 into one optical path. The thus synthesized RGB three-colors laser light from the lasers 1 to 3 is focused through a lens 9 to a pinhole 10. Divergent light leaving the pinhole 10 is incident on a photosensitive material 20 brought in close contact with a hologram plate 21 with an index-matching liquid filled between them. Then, the incident light and diffracted light from the hologram plate 21 interfere with each other in the photosensitive material 20, so that a color hologram having the same properties as those of the hologram plate 21 can be replicated.
When a multicolor hologram such as a full-color hologram is recorded or replicated with such a recording or replicating optical system as mentioned above, the color balance is well kept at the start of recording or replication. On completion of recording or replication, however, there is a problem that the color balance is disturbed.
Two or more laser beams are used for the recording, and replication of multicolor holograms as shown in FIGS. 6 and 7. However, the hues or tints of reconstructed images vary depending on the intensity ratio of laser light on the surface of the photosensitive material.
Although the color balance is well maintained at the start of recording or replication, it suffers from disturbance on completion of recording or replication, as already mentioned. The reason is that the distribution of each laser exposure intensity in the hologram surface varies during recording or replication, and so the ratio of each laser light intensity varies at each point in the hologram surface. The exposure intensity is strongest at the center of the optical axis of a laser beam, and becomes weak farther off the center. In other words, the change of the exposure intensity distribution in the hologram surface is tantamount to a deviation of the center of the optical axis of the laser beam from a given position. As holograms are actually recorded or replicated for a long period of time, for instance, one day or 12 hours, the center of a laser beam deviates from the center of the exposed surface. This appears to be due to the superposition of various reasons such as temperature changes.
Until now, this problem has been solved by manually measuring the exposure intensity distribution on the exposed surface, and then manually correcting the angle of each mirror in the optical system based on the measurement.
With this method, however, much time is needed for one regulation. Further, the exposure intensity distribution cannot be measured during, and simultaneously with, recording or replication.
Another problem with the recording or replication of a multicolor hologram such a full-color hologram using such a recording or replicating optical system as mentioned above is that there is a difference in the hues or tints of a reconstructed image between the central portion and the peripheral portion of the hologram surface. Such hue or tint variations on the hologram surface result from the rate of intensity decrease varying from the center to the periphery of each laser beam. Even when a hologram is recorded or replicated while the laser light of each color is balanced at the center of a photosensitive material, therefore, the balance suffers from disturbance at the periphery portion of the photosensitive material.
As already described, the fact that the rate of intensity decrease varying from the center to the periphery of each laser beam is due to a beam diameter difference between laser beams. A beam having a large diameter diverges widely with the rate of intensity decrease with respect to the distance from the center becoming small. This is in contrast to a beam having a small diameter.
So far, this problem has been solved not only by making exposure intensity at the center of the photosensitive material uniform but also by measuring exposure intensity at the central and peripheral portions of the exposure surface to find the maximum intensity balance at the central and peripheral portions of the exposed surface.
With this method, however, much time is needed because several measurements should be obtained in one regulation. As the exposure surface becomes wide, there is a large hue or tint difference between the central and peripheral portions of the exposed surface. According to this method, the hues or tints thus change unavoidably more or less in the surface.